In digital facsimile systems a document is scanned line by line to generate digitized data. Each bit of information, either a "1" or "0", corresponds to a small area on the document. The "color" of the area, i.e., whether the area is white or black, determines whether the associated binary signal is a "0" or a "1", respectively.
These lines of picture data are normally comprised of units of alternating color having various run lengths. For example, a line of picture data might have a white color unit of ten picture elements which represents a blank space in the document. This first color unit has a run length of ten. The next color unit is naturally black and may contain five picture elements for a run length of five. This color unit may represent part of a character on the document. Thus each line of picture data has an alternating sequence of white and black color units, each having a particular run length.
Since the straightforward transmission of this data may involve a large amount of digital information and may therefore consume much time, recent advances in the facsimile field have been made in the encoding of the information so that the amount of data which is required to be transmitted is compressed. These advances include the sequential coding of run lengths, color unit by color unit on a line, and line by line until the entire document is coded. This is called one-dimensional or run length coding. Another approach is the sequential coding of the positions of the first picture element of each color unit in a line to be coded with respect to the positions of the first picture element of color units in a line which has already been coded. This coding procedure, which is repeated line by line, is called two-dimensional coding. For an explanation of digital coding standards, see "International Digital Facsimile Coding Standards," by R. Hunter and A. H. Robinson in the Proceeding of the IEEE, Volume 68, No. 7, July 1980, pages 854-867.
The encoded data is transmitted to a distant location, received, and reformed to obtain a copy of the original document. The facsimile receiver has a decoding unit for decoding the compressed data into signals indicating the run lengths of successive color units. These run length signals are fed into a picture element generating unit which transforms the run length signals to picture element signals to reform the original document color unit by color unit and line by line. In an ideal transmission-reception there should be a one-to-one correspondence between the picture elements at the transmission end and the picture elements at the reception end.
The present invention is directed toward a high speed picture element generator. While prior art picture element generators typically form one picture element per clock cycle, the present invention is capable of generating eight picture elements per clock cycle. Generally the present invention generates any number of picture elements in a single color unit at one time, provided that the number is less than eight. With suitable modifications, as discussed below, a picture element generator unit according to this invention can generate even more picture elements per clock cycle. Thus the disclosed device has a significant advantage over devices in the prior art. Additionally, the president invention is suitable for VLSI (very large scale integration) fabrication in a semiconductor device.